High electron mobility field effect transistors (hereafter HEMT) have been used as an important component of radiofrequency communication equipment. The HEMT is characterized by the selectively doped heterostructure consisting of different materials for the electron-donating layer (dope layer) and the channel layer in which electrons travel. In this heterostructure, electrons supplied from n type impurities in the electron-donating layer collects in the potential wells formed on the channel side of the heterojunction interface due to the difference of electron affinity of materials constituting the heterojunction, resulting in the formation of two dimensional electron gas. Thus n type impurities supplying electrons are in the electron-donating layer, and because the electrons supplied from here separate ionization impurities spatially from electrons as they travel through the high-purity channel, the two dimensional electron gas in the channel is not scattered by ionization impurities and has high electron mobility.
While the HEMT is normally manufactured using an epitaxial substrate laminating thin film crystal layers having given electronic characteristics on a GaAs single crystal substrate so as to possess given structure, it is required to control the thin film crystal layer which forms the HEMT structure on the order of monoatomic layers so that the channel can have high electron mobility. Thus, to manufacture an epitaxial substrate having the HEMT structure, the molecular beam epitaxy (hereafter referred to as MBE) method or the metal-organic chemical vapor deposition (hereafter referred to as MOCVD) method has been used conventionally.
Of these methods, the MOCVD method, in particular, which involves using organometallic compounds or hydrides of atomic species constituting the epitaxial layer as raw materials and growing crystals on the substrate through thermal decomposition, has been widely used in recent years because of its wide applicability range and fitness for precise control of crystal composition and the film thickness.
III-V compound semiconductor materials widely used for these epitaxial substrates include GaAs and AlGaAs because they allow matching the lattice constant with given composition and various types of heterojunction are possible while keeping good crystallinity. However, because it is necessary to increase the electron mobility of the channel layer in order to improve the performance of HEMT, InGaAs has been used in recent years as a material for the channel layer instead of GaAs because it is not only superior in electron transport property but it can also change energy gaps dramatically according to the In composition and contain two dimensional electrons effectively. In addition, AlGaAs or GaAs can be used as a material to be combined with InGaAs.
Because lattice matching for GaAs was impossible, InGaAs could not be used formerly to obtain an epitaxial substrate having sufficient physical property. However, since reliable heterojunction was found possible even in the lattice mismatch system without causing a reduction in crystallinity such as dislocation if the mismatching is within the threshold of elastic deformation, efforts have been made towards practical use.
The threshold of film thickness of strained crystalline layer in such lattice mismatch system is given as a function of crystal layer composition, and in the case of the InGaAs layer for the GaAs layer, for example, the theoretical formula of Mathews is disclosed in J. Crystal Growth, 27 (1974), p. 118 and J. Crystal Growth, 32 (1976), p. 265. These theoretical formulas have been found almost correct in experiments.
In addition, JP-A-6-21106 discloses a technique to improve electron mobility by optimizing the In composition of the InGaAs strain layer and the film thickness of the InGaAs layer used for the channel layer of the p-HEMT structure using a given relational expression. Actually, an InGaAs layer with In composition of 0.20 and film thickness of about 13 nm has been put to practical use as an InGaAs strained channel layer that allows epitaxial growth without reducing crystallinity.
By using an epitaxial growth substrate configured to use such InGaAs layer for the channel layer part of conventional HEMT in which two dimensional electrons flow, electron devices have been fabricated that have higher mobility and superior noise characteristics compared to conventional ones. The HEMT using the InGaAs layer for the channel layer in which two dimensional electrons flow is referred to as a pseudomorphic high electron mobility transistor (hereafter pseudomorphic-HEMT or p-HEMT).
In p-HEMT, a layer called a space layer is usually formed between the strained channel layer, InGaAs layer, and the front side electron-donating layer as the layer to reduce the effect of impurity scattering due to the front side electron-donating layer on the electrons flowing in the channel layer. Furthermore, a layer to install a gate electrode of transistor generally referred to as a gate barrier layer or Schottky layer is formed on the surface side of the front side electron-donating layer. For these space layers and gate barrier layers, GaAs layers or AlGaAs layers have been used conventionally.
In addition, in p-HEMT, a GaAs or AlGaAs layer is usually formed as the electron-donating layer. However, an InGaP layer joined to a GaAs or AlGaAs layer in a lattice matching manner has been also used.
However, using a GaAs or AlGaAs layer for the space or gate barrier layer is problematic; GaAs has too small a band gap to allow gate withstand voltage for transistor gates, and AlGaAs has a problem in that incorporation of impurities results in the loss of crystallinity and surface state stability.
In addition, conventional p-HEMT structure required a layer rich in dopant as an electron-donating layer in order to achieve an amount of two dimensional electron gas required in the channel layer to improve the current value of transistors. However, for the reasons described above, it was difficult to further improve transistor performance because the crystallinity of the electron-donating layer decreased due to excess dopant and the withstand voltage of the gate decreased.
As means of solving these problems, a configuration designed to lower the dopant concentration of the front side electron-donating layer and thicken its film thickness, or in the case of a double hetero structure, a configuration designed to lower the dopant concentration of the front side electron-donating layer and increase the dopant concentration of the back side has been proposed.
However, even if the configuration proposed above is employed in an epitaxial substrate of p-HEMT structure, it is difficult to employ an electron-donating layer with a low dopant concentration to achieve a high two dimensional electron gas concentration and obtain an epitaxial substrate of p-HEMT structure having good transistor characteristics such as pinch off characteristics if GaAs or AlGaAs is used for the gate barrier layer.
In this view, for the p-HEMT used for various mobile equipment such as cell phones, improving gate withstand voltage and pinch off characteristic is required, and it is necessary to use an electron-donating layer with a low dopant concentration to increase two dimensional electron gas concentration to improve the characteristics of electronic devices. However, the above described conventional technologies are not sufficient to meet these needs.